In born errors in the biosynthesis of the pterin cofactors tetrahydrobiopterin (H4B) and molybdopterin (MoCo) are associated with serious human diseases. Genetic variations in the synthesis of H4B have been associated with serious human diseases. Genetic variations in the synthesis of H4B have been associated with diseases such as atypical phenylketonuria, Parkinson's disease, and dystonias. The importance of H4B to these processes is found in its role as an essential cofactor for phenylalanine, tyrosine and tryptophan hydroxylases, enzymes that are involved in amino acid degradation and the biosynthesis of catecholamines and serotonin. The MoCo functions as an essential enzymatic cofactor for molybdenum hydroxylase enzymes such as sulfite oxidase and xanthine dehydrogenase. Genetic deficiencies in the biosynthesis of the MoCo ar3e associated with a severely debilitating neurological disorder, molybdenum cofactor deficiency, and with xanthinuria, a malady of purine metabolism. The proposed studies utilize Drosophila melanogaster, the only model system in which changes in pterin synthesis are reflected in a visible phenotype (eye color), as a genetic model system for pterin cofactor biosynthesis. The studies center on two Drosophila proteins, aldose reductase and the maroon-like+ (ma- 1+) protein, which are involved, respectively, in the synthesis of H4B and MoCo. Expressed sequence tags (ESTSs) identified from partial amino acid analysis will be used to obtain molecular clones encoding the aldose reductase. Aldose reductase clones will be used as probes to determine the cytogenetic locus of the aldose reductase gene(s). Knowledge of the cytogenetic locus of the aldose reductase will allow mutant analysis to assess the protein's involvement in the H4B pathway. Polyclonal antisera raised to maroon-like (ma-1) peptides will be used to follow the standard purification of the maroon-like protein, a putative cysteine transulfurase, and the first eukaryotic member of the nif-S-like gene family to be described. Such antibodies will also be used to localized the ma-1 protein in situ and to analyze existing ma-1 mutants. These studies will lead to a better understanding of the terminal steps of BH4 and MoCo biosynthesis in higher organisms.